What is the primary ganglion involved in defining a postganglionic Horner's syndrome?

The primary ganglion involved in postganglionic Horner's syndrome is the superior cervical ganglion. This condition arises from disruption of sympathetic nerve pathways that supply the eye and the surrounding structures.

Horner's syndrome results from damage to the sympathetic nerve supply, which can occur at various points along the pathway involved in the sympathetic innervation to the eye. The superior cervical ganglion is the site where the postganglionic fibers arise and innervate structures such as the pupil (causing miosis), the eyelid (leading to ptosis), and potential anhidrosis on the affected side of the face.

In contrast, other ganglions listed serve different roles. The inferior cervical ganglion is primarily involved in sympathetic innervation to the heart and neck, while the paravertebral ganglia are part of the sympathetic trunk but do not directly relate to the postganglionic innervation of the eye. The celiac ganglion is involved in innervating abdominal organs and is unrelated to Horner's syndrome.

Thus, the superior cervical ganglion is critically linked to the presentation of postganglionic Horner's syndrome due to its direct involvement in the sympathetic innervation of ocular structures.